The present invention relates to 2,6-di-tert-butylphenols of the formula I 
in which
R is H, an alkyl or alkenyl radical having 1 to 18 carbon atoms which is unsubstituted or substituted by CN or by at least one halogen, e.g., up to perhalo-substituted, and in which one or more non-adjacent CH2 groups may be replaced by a radical selected from the group consisting of xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94 and xe2x80x94Cxe2x89xa1Cxe2x80x94,
A1 and A2 are, independently of one another,
a) a 1,4-phenylene radical, in which one or two CH groups may be replaced by N,
b) a 1,4-cyclohexenylene or 1,4-cyclohexylene radical, in which one or two non-adjacent CH2 groups may be replaced by xe2x80x94Oxe2x80x94 or xe2x80x94Sxe2x80x94,
c) a piperidine-1,4-diyl, a 1,4-bicyclo[2.2.2]-octylene or naphthalene-2,6-diyl radical,
where the radicals a) and b) may be monosubstituted or polysubstituted by halogen atoms,
Z1 and Z2 are each, independently of one another, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94CH2xe2x80x94Oxe2x80x94, xe2x80x94OCH2, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CHxe2x95x90CRxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94 or a single bond, and
m and n are each, independently of one another, 0, 1, 2 or 3, where m+nxe2x89xa71,
with the proviso that compounds of the formulae 
are excluded.
Since autoxidation can result in a considerable drop in value of many natural products, for example foodstuffs and natural rubber, and, industrial products, for example monomers for polymerization purposes, petroleum products and plastics, the search for substances which prevent or retard autoxidation in the lowest possible concentration has become a broad area of research.
The phenolic antioxidants used today in industry are phenols which are substituted in the ortho-position to the hydroxyl group, usually by bulky groups. The action of these sterically hindered phenols is based on the ready liberation of the phenolic hydrogen atom with formation of phenoxide free radicals.
The use of antioxidants relates essentially to the stabilization of oils, fats and plastics. Antioxidants for these purposes must not be toxic, should have high effectiveness at low concentrations, should not cause any changes in taste, odor or color, even on extended storage or heating, should be readily soluble or easily dispersible in the substrate, should not exert any adverse effect on the substance to be protected, should have low volatility, and should allow analytical determination as quantitatively as possible.
Furthermore, antioxidants should be simple and inexpensive to prepare, and their handling and incorporation should be simple.
The prior art contains phenolic antioxidants of the formulae 
However, the antioxidants mentioned do not satisfy all requirements. A particular disadvantage is that the antioxidants mentioned must be employed in relatively high concentrations.
An object of the present invention was therefore to find inexpensive antioxidants which satisfy all the conditions mentioned.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
Surprisingly, it has now been found that the 2,6-di-tert-butylphenols of the formula I have high effectiveness as antioxidants, even at low concentrations, without exerting any adverse effect on the substance to be protected. Furthermore, the novel sterically hindered phenols are distinguished by heat resistance, low viscosity and low volatility.
The invention thus relates to 2,6-di-tert-butylphenols of the formula I.
The claimed compounds are free-radical scavengers and are therefore suitable for suppressing ageing processes caused by free-radical chain reactions, such as, for example, autoxidation.
Owing to their structure, these materials have a particularly low vapor pressure and can thus also be employed at high temperatures. They and their reaction products are colorless and thus do not result in any discoloration of the materials to be stabilized.
For reasons of simplicity, phenol below denotes 
Cyc denotes a 1,4-cyclohexylene radical, Che denotes a 1,4-cyclohexenylene radical, Dio denotes a 1,3-dioxane-2,5-diyl radical, Dit denotes a 1,3-dithiane-2,5-diyl radical, Phe denotes a 1,4-phenylene radical, Pyd denotes a pyridine-2,5-diyl radical, Pyr denotes a pyrimidine-2,5-diyl radical and Bi denotes a bicyclo[2.2.2]octylene radical, where Cyc and/or Phe may be unsubstituted or monosubstituted or disubstituted by F.
The compounds of the formula I accordingly include bicyclic compounds of the subformulae Ia and Ib:
tricyclic compounds of the subformulae Ic to Ig:
and tetracyclic compounds of the subformulae Ih to Io:
Of these, particular preference is given to those of the subformulae Ia, Ib, Ic, Id, Ie and If.
In the compounds of the formula I, Z1 and Z2 are preferably a single bond or CH2CH2, secondarily preferably xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94 or xe2x80x94COxe2x80x94Oxe2x80x94.
If one of the radicals Z1 and Z2 is xe2x80x94(CH2)4xe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94CH2CHxe2x80x94, the other radical Z1 or Z2 (if present) is preferably a single bond.
The sun n+m is preferably 1 or 2.
If R is an alkyl radical and/or an alkoxy radical, this can be straight-chain or branched. It is preferably straight-chain, has 2, 3, 4, 5, 6 or 7 carbon atoms and accordingly is preferably ethyl, propyl, butyl, pentyl, hexyl,. heptyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy or heptoxy, furthermore methyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, methoxy, octoxy, nonoxy, decoxy, undecoxy, dodecoxy, tridecoxy or tetradecoxy.
Oxaalkyl, i.e., having a CH2 group replaced by xe2x80x94Oxe2x80x94, is preferably straight-chain 2-oxapropyl (=methoxymethyl), 2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or 4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl, or 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.
If R is an alkyl radical in which one CH2 group has been replaced by xe2x80x94CHxe2x95x90CHxe2x80x94, this can be straight-chain or branched. It is preferably straight-chain and has 2 to 10 carbon atoms. Accordingly, it is in particular vinyl, prop-1- or -2-enyl-, but-1-, -2- or -3-enyl, pent-1-, -2-, -3- or -4-enyl, hex-1-, -2-, -3-, -4- or -5-enyl, hept-1-, -2-, -3-, -4-, -5- or -6-enyl, oct-1-, -2-, -3-, -4-, -5-, -6- or -7-enyl, non-1-, -2-, -3-, -4-, -5-, -6-, -7- or -8-enyl, or dec-1-, -2-, -3-, -4-, 5-, -6-, -7-, -8- or -9-enyl.
If R is an alkyl radical in which one CH2 group has been replaced by xe2x80x94Oxe2x80x94 and one has been replaced by xe2x80x94COxe2x80x94, these are preferably adjacent. These thus contain one acyloxy group xe2x80x94COxe2x80x94Oxe2x80x94 or one oxycarbonyl group xe2x80x94Oxe2x80x94COxe2x80x94. These are preferably straight-chain and have 2 to 6 carbon atoms.
Accordingly, they are in particular acetoxy, propionyloxy, butyryloxy, pentanoyloxy, hexanoyloxy, acetoxymethyl, propionyloxymethyl, butyryloxymethyl, pentanoyloxymethyl, 2-acetoxyethyl, 2-propionyloxyethyl, 2-butyryloxyethyl, 3-acetoxypropyl, 3-propionyloxypropyl, 4-acetoxybutyl, methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl, ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxycarbonylmethyl, 2-(methoxycarbonyl)ethyl, 2-(ethoxycarbonyl)ethyl, 2-(propoxycarbonyl)ethyl, 3-(methoxycarbonyl)propyl, 3-(ethoxycarbonyl)propyl or 4-(methoxycarbonyl)butyl.
If R is an alkyl radical in which one CH2 group has been replaced by unsubstituted or substituted xe2x80x94CHxe2x95x90CHxe2x80x94 and an adjacent CH2 group has been replaced by CO or COxe2x80x94O or Oxe2x80x94COxe2x80x94, this can be straight-chain or branched. It is preferably straight-chain and has 4 to 13 carbon atoms. Accordingly, it is in particular acryloyloxymethyl, 2-acryloyloxyethyl, 3-acryloyloxypropyl, 4-acryloyloxybutyl, 5-acryloyloxypentyl, 6-acryloyloxyhexyl, 7-acryloyloxyheptyl, 8-acryloyloxyoctyl, 9-acryloyloxynonyl, 10-acryloyloxydecyl, methacryloyloxymethyl, 2-methacryloyloxyethyl, 3-methacryloyloxypropyl, 4-methacryloyloxybutyl, 5-methacryloyloxypentyl, 6-methacryloyloxyhexyl, 7-methacryloyloxyheptyl, 8-methacryloyloxyoctyl and 9-methacryloyloxynonyl.
If R is an alkyl or alkenyl radical which is at least monosubstituted by halogen, this radical is preferably straight-chain and halogen is preferably F or Cl. In the case of multiple substitution, halogen is preferably F. The resultant radicals also include perfluorinated radicals. In the case of monosubstitution, the fluorine or chlorine substituent can be in any desired position, but is preferably in the xcfx89-position.
Branched radicals R generally contain not more than one chain branch. Preferred branched radicals R are isopropyl, 2-butyl (=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl (=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl, 2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy, 3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy, 1-methylhexoxy and 1-methylheptoxy.
If R is an alkyl radical in which two or more CH2 groups have been replaced by xe2x80x94Oxe2x80x94 and/or xe2x80x94COxe2x80x94Oxe2x80x94, this may be straight-chain or branched. It is preferably branched and has 3 to 12 carbon atoms. Accordingly, it is in particular biscarboxymethyl, 2,2-biscarboxyethyl, 3,3-biscarboxypropyl, 4,4-biscarboxybutyl, 5,5-biscarboxypentyl, 6,6-biscarboxyhexyl, 7,7-biscarboxyheptyl, 8,8-biscarboxyoctyl, 9,9-biscarboxynonyl, 10,10-biscarboxydecyl, bis(methoxycarbonyl)methyl, 2,2-bis(methoxycarbonyl)ethyl, 3,3-bis(methoxycarbonyl)propyl, 4,4-bis(methoxycarbonyl)butyl, 5,5-bis(methoxycarbonyl)pentyl, 6,6-bis(methoxycarbonyl)hexyl, 7,7-bis(methoxycarbonyl)heptyl, 8,8-bis(methoxycarbonyl)octyl, bis(ethoxycarbonyl)methyl, 2,2-bis(ethoxycarbonyl)ethyl, 3,3-bis(ethoxycarbonyl)propyl, 4,4-bis(ethoxycarbonyl)butyl and 5,5-bis(ethoxycarbonyl)pentyl.
Preferred subgeneric groups of compounds of the formula I are those of the subformulae I1 to I10 
Particular preference is given to the compounds of the formulae I1 and I2.
The compounds of the formula I are prepared by methods known per se, as described in the literature (for example in the standard works such as Houben-Weyl, Methoden der Organischen Chemie, Georg Thieme Verlag, Stuttgart), to be precise under reaction conditions which are known and suitable for said reactions. Use can also be made here of variants which are known per se, but are not mentioned here in greater detail.
The novel compounds can be prepared, for example, in accordance with Schemes 1 and 2: 
The compounds of the formula I have a broad range of applications. They can easily be used as stabilizers in the foods sector and in industrial products.
The invention also relates to the use of the novel compounds as additives for liquid-crystal mixtures, liquid-crystalline individual components, lubricants, heat-transfer media, fuels, pharmaceutical products, vitamin preparations, cosmetics, foodstuffs, dyes, paints, polymers, elastomers, detergents, solvents, paper and crop-protection agents.
The compounds of the formula I are added to the material to be stabilized in concentrations of from 0.01 to 5% by weight, preferably from 0.01 to 2% by weight, in particular from 0.1 to 1% by weight.
Owing to their rod-like structure, the compounds of the formula I are particularly suitable as stabilizers for liquid-crystalline individual components and liquid-crystal mixtures, since they interfere with the liquid-crystalline properties much less than do the anti-oxidants from the prior art.
Since the novel compounds have a particularly low vapor pressure, they can also be employed at high temperatures. They have properties similar to liquid crystals and do not cause discoloration in liquid-crystal mixtures. In contrast to known antioxidants, they do not have an adverse effect on the clearing point of the liquid-crystalline individual components.
The stabilization of the liquid crystals or liquid-crystal mixtures is carried out in a manner known per se. In general, all components are dissolved in one another, preferably at elevated temperature.
The novel stabilizers are preferably employed in STN liquid crystal mixtures containing tolans and alkenyls. The compounds of the formula I can also be added to the individual components, preferably alkenyls and tolans.
In particular, the novel stabilizers are suitable for suppressing decomposition and polymerization products of the following compounds. 
In such compounds, the double bond reacts particularly quickly with atmospheric oxygen. In particular, a double bond between two cyclohexane rings is sensitive to oxygen compared with a double bond in a side chain. The addition of from 0.01 to 5% by weight, in particular from 0.01 to 2% by weight, of a novel stabilizer, suppresses ageing processes initiated by O2 free radicals from the air.
The invention also relates to a liquid-crystalline medium comprising at least two liquid-crystalline compounds and additionally a 2,6-di-tert-butylphenol of the formula I.
The invention furthermore relates to stable liquid-crystalline mixtures comprising at least one compound containing one or more double bonds and a stabilizer of the compound I.
The novel liquid-crystalline medium generally consists of from 2 to 25, preferably from 3 to 15, LC components and a stabilizer of the formula I. However, it is also possible in individual cases for more than 25 components to be constituents of a novel phase, for example up to 50 components or more. The other constituents are preferably selected from nematic or nematogenic substances, in particular known substances, from the classes consisting of the azoxybenzenes, benzylideneanilines, biphenyls, terphenyls, phenyl or cyclohexyl benzoates, phenyl or cyclohexyl cyclohexanecarboxylates, phenylcyclohexanes, cyclohexylbiphenyls, cyclohexylcyclohexanes, cyclohexylnaphthalenes, 1,4-bis-cyclohexylbenzenes, 4,4xe2x80x2-bis-cyclohexylbiphenyls, phenyl- or cyclohexylpyrimidines, phenyl- or cyclohexyldioxanes, phenyl- or cyclohexyldithianes, 1,2-diphenylethanes, 1,2-dicyclohexylethanes, 1-phenyl-2-cyclohexylethanes, optionally halogenated stilbenes, benzyl phenyl ethers, tolans and substituted cinnamic acids.
The most important compounds which are suitable as constituents of liquid-crystalline phases of this type can be characterized by the formula II
R1xe2x80x94Lxe2x80x94Gxe2x80x94Exe2x80x94R2xe2x80x83xe2x80x83II
in which L and E are each a carbocyclic or heterocyclic ring system from the group consisting of 1,4-disubstituted benzene and cyclohexane rings, 2-fluoro-1,4-phenylene, 3-fluoro-1,4-phenylene, 2,6-difluoro-1,4-phenylene, 4,4xe2x80x2-disubstituted biphenyl, phenylcyclohexane and cyclohexylcyclohexane systems, 2,5-disubstituted pyrimidine and 1,3-dioxane rings, 2,6-disubstituted naphthalene, di- and tetrahydronaphthalene, quinazoline and tetrahydroquinazoline,
G is xe2x80x94CHxe2x95x90CHxe2x80x94
xe2x80x94CH=CYxe2x80x94
xe2x80x94Cxe2x89xa1Cxe2x80x94
xe2x80x94COxe2x80x94Oxe2x80x94
xe2x80x94COxe2x80x94Sxe2x80x94
xe2x80x94CH=Nxe2x80x94
xe2x80x94N(O)=Nxe2x80x94
xe2x80x94CH=N(O)xe2x80x94
xe2x80x94CH2xe2x80x94CH2xe2x80x94
xe2x80x94CH2xe2x80x94Oxe2x80x94
xe2x80x94CH2xe2x80x94Sxe2x80x94
xe2x80x94COOxe2x80x94Phexe2x80x94COOxe2x80x94
or a Cxe2x80x94C single bond, where Y is halogen, preferably chlorine, or xe2x80x94CN, and R1 and R2 unsubstituted or substituted by at least one or more fluoro atoms alkyl, alkoxy, alkanoyloxy or alkoxycarbonyloxy having 1 to 18, preferably 1 to 8, carbon atoms, or one of these radicals is alternatively CN, NO2, CF3, F, Cl or Br.
In most of these compounds, R1 and R2 are different from one another, one of these radicals usually being an alkyl or alkoxy group, and the other being alkyl, alkenyl, alkoxy, CN, F, Cl, OCF3, OCHF2, OCH2CF3, OCHFCF3, OCF2CHFCF3, OCH=CF2, OCF=CF2 or CH=CF2. However, other variants of the proposed substituents are also common. Many such substances or mixtures thereof are commercially available. All these substances can be prepared by methods which are known from the literature.
In the foregoing and in the following examples, all temperatures are set forth uncorrected in degrees Celsius; and, unless otherwise indicated, all parts and percentages are by weight.
The entire disclosure of all applications, patents and publications, cited above and below, and of corresponding German application No. 195 39 141.1, filed Oct. 20, 1995 is hereby incorporated by reference.
In the present application and in the examples below, the structures of the liquid-crystal compounds are indicated by acronyms, with the transformation into chemical formulae taking place in accordance with Tables A and B below. All radicals CnH2n+1 and CmH2m+1 are straight-chain alkyl radicals having n or m carbon atoms respectively. The coding in Table B is self-evident. In Table A, only the acronym for the parent structure is given. In individual cases, the acronym for the parent structure is followed, separated by a dash, by a code for the substituents R1, R2, L1 and L2:
Preferred mixture components are shown in Tables A and B.